ELECTROCHEMISTRY OF CIS-1,4,8,11-TETRAAZACYCLOTETRADECANE AND TRANS-1,4,8,11-TETRAAZACYCLOTETRADECANE COMPLEXES OF COBALT(III) AT GOLD ELECTRODES IN HYDROXIDE SOLUTIONS

The electrochemistry of trans- and cis-dihydroxycobalt(III) 1,4,8,11-tetraazacyclotetradecane ([14]aneN4) complexes has been studied in 3 M NaOH at gold electrodes. A square electron-transfer scheme is found to be operative that incorporates electron-induced cis/trans isomerization via Co(II)cyclam intermediates. Attention is focused on the role of gold surface interactions with the CO(III)cyclam complexes and on the electron-induced isomerization of cis-[Co(III)([14]aneN4)(OH)2]+ complex. A consistent set of thermodynamic and kinetic parameters is obtained by a combination of cyclic voltammetric and chronoamperometric experiments. The analysis indicates that the cis- and trans-Co(II)cyclam isomers have nearly equal energies, the trans isomer being more stable by 0.6 kJ/mol, while the trans-[Co(III)([14]aneN4)(OH)2]+ isomer is more stable than the cis-[Co(III)([14]aneN4)(OH)2+] isomer by 21.3 kJ/mol. At room temperature the lifetime of cis-[Co(II)([14]aneN4)(OH2)2]3+ (tau = 0.693/k-2, where k-2 is the first-order rate constant for isomerization) is 2.6 s, 1500 times more short-lived than the cis-[Co(III)([14]aneN4)(OH)2]+ complex.